Sueo Maruo, known as Maruo-kun, is a third-grade student with a signature bowl-cut hairstyle featuring straight-across bangs above his eyebrows. His Picasso-like eyes, framed by double eyelids and prominent lashes, peer through thick milk-bottle glasses. Dark green dominates his wardrobe: he typically sports a dark-green long-sleeve shirt revealing white undershirt collar and cuffs, paired with dark blue or black shorts, white socks, and green shoes. Summer swaps long sleeves for short ones but keeps the white cuffs, maintaining dark green as his signature color through stripes or ties.

As class president, he campaigns energetically to retain his position, crafting handmade posters and delivering megaphone speeches. Academically, he excels in diligence and tops most subjects, though struggles in physical education and music. Outward confidence masks emotional vulnerability; he craves peer affirmation, often inserting contradictory opinions into conversations to gain attention—a tactic that occasionally backfires. Criticism easily provokes tears, especially when his reputation is threatened, such as classmates vowing not to vote for him.

He adores his mother, sometimes dedicating achievements to her, and visibly blushes or beams at praise. Despite enthusiasm, his intrusive tendencies limit appreciation from female classmates, though he relishes any affection shown. His catchphrase "Zubari!" punctuates conversations. Favorite foods include soba noodles, loquats, tempura, omelettes, and kashiwa-mochi, while natto is disliked. His class president badge is his most prized possession, and he cleans his ears almost daily.

In a film context hosting international homestay students, he participates alongside peers like Maruko and Hanawa, leveraging his recognition-seeking and emotionally responsive traits during group activities. His family includes his mother (real name unspecified) and a mentioned father.